The continuous shrink of the critical sizes in modern Integrated Circuit (IC) features associated with 3D topology architectures offers highest density at the expense of process complexity.
According to the International Technology Roadmap for Semiconductors (ITRS), physical techniques commonly used in the semiconductor industry for the deposition of thin films are no more suitable to meet the requirements in the future technology node, notably for high aspect ratio structures. Techniques like PVD (Physical Vapor Deposition), i-PVD (ionized-Physical Vapor Deposition) or PECVD (Plasma-Enhanced Chemical Vapor Deposition), employing high energy particles, induce high sticking coefficient which leads to poor step coverage, especially along the sidewalls.
The main industrial options to enable the deposition of highly uniform and conformal thin films with reasonable throughput in high aspect ratio structures are techniques such as MOCVD (Metal-Organic Chemical Vapor Deposition) or ALD (Atomic Layer Deposition).
However, films deposited by MOCVD need high thermal budget and generally follow a 3D-growth mechanism described by a Volmer-Weber model. Thin films grow by clusters nucleation and such technique leads to insufficient step coverage.
The typical ALD process (e.g as described in RITALA M., LESKELA M., Atomic Layer Deposition, Handbook of thin films materials) involves gaseous reactants led onto a substrate by pulses, separated by inert gas purging. In MOCVD, gaseous reactants are injected simultaneously and react by thermal self-decomposition while in ALD; the loss of the ligand is thermally induced by reaction with the surface groups on the substrate. In a temperature range, the surface reactions are self-limited, which allow the deposition of highly uniform and conformal films. Precursors must be volatile and stable enough to be easily transferred to the reaction chamber without being decomposed.
Moreover, they must be reactive enough with the chemical groups of the surface to ensure reasonable growth rate.
ALD is of particular interest for the deposition of group V (V, Nb, Ta) metal containing films. Today, there still exist the needs for metal organic precursors in liquid state at room temperature (or close to room temperature) having a high volatility and having a high versatility: suitable for various applications in the semi-conductor manufacture. Interest for conductive (resistivity <1000 μΩ·cm) group V (V, Nb, Ta) metal containing thin films deposited by ALD has risen in the past few years for several main applications such as: copper diffusion barrier in BEOL applications, CMOS metal gate, electrodes for Metal-Insulator-Metal applications (DRAM . . . ), and/or the like in TFT-LCD applications.
Group V (V, Nb, Ta) metal containing films are also of particular interest for High-k layers in memory devices
Halides such as CpNbCl4 (CAS 33114-1507), NbF5, NbBr5 (Thin solid films, 1981, 79, 75), NbCl5 (Crystal growth, 1978, 45, 37) and such as TaCl5, disclosed in U.S. Pat. No. 6,268,288, have been widely investigated. However, some by-products generated during the deposition process, such as HCl or Cl2, can cause surface/interface roughness that can be detrimental to the final properties. Moreover, Cl or F impurities can be detrimental to the final electrical properties. It is therefore expected to find new compounds having sufficient volatility but without containing Cl, F, or Br atoms.
Many Group V precursors have been considered to enable such deposition. Examples can be given as follows:
Alkoxides such as penta-ethoxy-Tantalum (PET) are widely used and described. However, they lead to oxygen containing films and are not suitable for the deposition of metal containing films which are used in particular as electrodes and which should not contain oxygen even at trace level. The same problem is observed for compounds such as Cp2Nb(H)(CO), CpNb(CO)4 (J. Organomet. Chem 557(1998)77-92), V(CO)6 (Thermochimica Acta, 1984, 75, 71), (η5-C5H5)V(CO)4 (M. L. Green, R. A. Levy, J. Metals 37 (1985) 63).
U.S. Pat. No. 6,379,748 discloses an improvement to PET. Alkyl bonds have been introduced, e.g. by using TaMe3(OEt)2 instead of Ta(OEt)5 (PET). Volatility was thereby significantly improved without affecting the melting point.
However, TaMe3(OEt)2 does not allow versatile deposition: in particular, oxygen free metal cannot be obtained.
U.S. Pat. No. 6,368,398 discloses another improvement with the use for instance of Ta[OC(O)C(CH3)3]5, however with the same limitation as disclosed here above.
WO 02/20870 discloses the use of tert-butylimido(tris(diethylamido) Tantalum, TBTDET, for the deposition of Ta2O5.
U.S. Pat. No. 6,593,484, US 2004/0219784 disclose a method of deposition of Tantalum nitride films by sequential injection of TBTDET or TAIMATA and other N source.
U.S. Pat. No. 6,379,748 discloses Ta(Me3SiCp)2H3, which is a biscyclopentadienyl Ta hydride which is a solid having a low volatility.